Axis of symmetry of parabola

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I have equation of parabola $(ax+by)^2+2fy=0$ and I have to find axis of this parabola so I made the substitution $X = ax +by$ and $Y= \frac{x}{a}-\frac{y}{b}$ and then solving by these substitution I m getting axis of symmetry $$ax+by = \frac{-bf}{a^2+b^2}$$ this is correct answer according to book. but the problem is I have made random substitution to bring the answer and I do not know why this substitution works and I also want to know how can we approach to find axis of symmetry for more general case $$(ax+by)^2+2gx+2fy+c=0$$

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This page tells you how to find the angle of rotation. As the description indicates, the same technique can be applied to any conic section (i.e. to any second degree equation), not just an equation representing a parabola.

As one of the comments suggested, the basic idea is to write your equation using a coordinate system that is rotated by some (as yet unknown) angle $\theta$. Then you choose the value of $\theta$ that makes the coefficient of the $xy$ term zero.